This invention relates to improvement of a hologram, and more particularly to a transparent-type hologram which is itself transparent and yet is also endowed with the characteristics of a reflection type hologram.
According to one method of recording a hologram, an interference fringe corresponding to the waveface of a light from a body of matter is recorded with an uneven pattern of the material surface, and the relief hologram obtained according to method has the advantage of performing a large amount of duplications easily by a press technique.
In these days, various attempts have been made to form relief holograms on substrates such as a credit card, cash card, book cover, record jacket, etc. When such a relief hologram is made integral with a substrate by the method such as transfer, plastering, direct formation, etc., the reflected light intensity becomes an important problem. Since a relief hologram is generally made of a synthetic resin, the reflected intensity at the relief forming surface, namely at the interface having a refractive index difference between the resin and air is small, and therefore the holographic effect is small.
For example, when a relief hologram is formed with a resin having a refractive index: n=1.5, the reflectance at the relief forming face R is very small (R.apprxeq.4% according to the Fresnel formula) and the holographic effect is insufficient.
Accordingly, in the prior art, it has been devised to make greater the reflected light intensity by forming a thin reflective metal film having a reflectance of 90% or higher (e.g. thin aluminum film) by vapor deposition, etc., on the relief forming face, thereby to exhibit sufficient holographic effect, and most of the so-called reflection type holograms employ such a structure.
However, in the hologram of the prior art comprising a thin reflective metal film formed on the relief forming face, even if a picture pattern layer may be formed by printing on the back of the thin metal film, no picture pattern layer can be seen because the lightwave propagated through the thin metal film will be attenuated in the progress direction, and therefore it is impossible to improve the appearance from aspect of design at the hologram forming face,. Thus uses are limited.
Supports such as cards, book covers, etc., are usually applied with a display portion such as letters, picture patterns, etc., on the surface thereof, and in this case the position where the hologram is formed is a problem. Formerly, it has been generally practiced to provide a hologram at a portion where no display portion such as letters, picture patterns, etc., is applied. This is because the hologram provided on a support such as cards is generally the reflection type hologram and the hologram itself lacks transparency. Therefore provision of a hologram on the display portion such as letters will obstruct visual observation of the display portion of letters, etc., to reduce its display effect.
Accordingly, in the prior art, the hologram must but be provided at the portion where no display portion is applied, and therefore there have been drawbacks such as (1) the position where the hologram is to be formed is limited, and the degree of freedom in designing was lacking, (2) accordingly, depending on the manner of arrangement or the size of the display portion applied on the support, formation of the hologram can be done with difficulty or partial reduction of the display effect of the display portion must but be tolerated, and (3) while it is desirable to form a hologram on the display portion such as a numerical representation, magnetic recording portion, sealing portion, etc., for preventing forgery of cards or securities, it can hardly be realized for the above reasons.
Further, a reflective metal film is usually colored in silver white and is also high in luminance, and therefore there has been the problem of unaesthetic appearance when a hologram is provided on various kinds of supports such as cards, etc.